This invention relates to a valve for controlling flow rate or pressure as a function of electric and other input signals, or in other words to a servo valve.
The nozzle-flapper type valve has been common as a conventional electric-pneumatic servo valve. However, such a nozzle-flapper type pneumatic servo valve has the following shortcomings.
(1) Its mechanical structure is so complicated that it causes difficulty for compact design, and its manufacturing cost is extremely high.
(2) The response is delayed due to the nozzle flapper system and other mechanical moving parts.
Lately, in Japanese Patent Laying-Open Publication No. 58-161821, an electric-pneumatic servo valve as briefly shown in FIG. 1 has been proposed. In this drawing, the reference numeral 1 denotes a main body, 2 denotes a first chamber formed within the main body 1, and 3 denotes a second chamber which is also formed within the main body 1. 4 is a source pressure valve outlet which opens into the first chamber 2 and receives a source pressure P0 from outside, while 5 is a valve element which cooperates with the source pressure valve outlet 4, and the source pressure valve outlet 4 and the valve element 5 make up a source pressure valve 6. 7 is an armature of the source pressure valve 6, mounted on the valve element 5.
8 is a pressure relief valve outlet which communicates the first chamber 2 with the second chamber 3, while 9 is a valve element which cooperates with the pressure relief valve outlet 8, and the pressure relief valve outlet 8 and the valve element 9 make up a pressure relief valve 10. 11 is an armature of the pressure relief valve 10, mounted on the valve element 9, while 12 is a gain adjustment spring which, interposed between the two armatures 7, 11, biases the valve element 5 so as to close the source pressure valve outlet 4 and at the same time biases the valve element 9 so as to open the pressure relief valve outlet 8.
13 is a zero adjustment spring which biases the valve element 9 so as to close the pressure relief valve outlet 8, 14 is an opening which communicates the second chamber 3 with the atmosphere, 15 is an output port which opens out from the first chamber 2, and 16 is an electromagnetic coil.
Now, suppose a fixed source pressure P0 is always supplied to the source pressure valve outlet 4 from outside, then according to the equilibrium of forces acting on the valve element 5 of the source pressure valve 6 the following equation holds: EQU PoA+f.ltoreq.PA+k1x1 (1)
where A is the effective cross sectional area of the source pressure valve outlet 4 and the pressure relief valve outlet 8, f is the electromagnetic attracting force between the armatures 7 and 11, P is the output pressure obtained at the output port 15, k1 is the spring constant of the gain adjustment spring 12, and x1 is the displacement of the gain adjustment spring 12.
And according to the equilibrium of forces acting on the valve element 9 of the pressure relief valve 10, the following equation holds: EQU PA+k1x1.ltoreq.k2x2+f (2)
where k2 is the spring constant of the zero adjustment spring 13, and x2 is the displacement of the zero adjustment spring 13.
The above two equations are equations of inequality instead of equations of equality because the source pressure valve 6 and the pressure relief valve 10 are one way valves. Now, the output pressure P obtained at the output pressure outlet 15 takes a value which satisfies both Eqs. (1) and (2), but by adjusting the initial displacement of the springs 12, 13 with an adjustment mechanism not shown in the drawing so that EQU P0A=k1x1=k2x2 (3)
holds, then the solution to the set of equations of inequality consisting of Eqs. (1) and (2) is given as: EQU P=f/A (4)
As can be seen from this equation, by controlling the electric current supplied to the coil 16 and thereby changing the magnetic attractive force F acting between the armatures 7 and 11, the output pressure P may be controlled.
This pneumatic servo valve can alleviate the shortcomings of the previously mentioned nozzle-flapper type pneumatic servo valve to a certain extent. However, even according to this pneumatic servo valve, there are such shortcomings as:
(a) The mechanical structure is still complicated, and the number of component parts is great. (FIG. 1 is intended for the explanation of the working principles of the servo valve, and is therefore extremely simpified. Although the number of parts appearing in FIG. 1 is not very large, in reality the number of parts of this servo valve cannot help but be great, because of the adjustment mechanism for adjusting the initial displacements of the springs 12 and 13, among other things).
(b) The response speed is not yet sufficient.
(c) The mechanical adjustment of the gain adjustment spring 12 and the zero adjustment spring 13 is subtle and requires some skill.
(d) For the above reason, some risk of increasing the variations in the gain of each servo valve may be caused.
(e) There may be some risk that the armatures 7 and 11 may adhere to each other whereby the servo valve may be put out of operation.
(f) When the source pressure P0 changes, the mechanical adjustment of the gain adjustment spring 12 and the zero adjustment spring 13 must be made all over again.
These shortcomings (a) to (f) are in principle caused by the fact that the source pressure valve 6 and the pressure relief valve 10 are connected together by a mechanical system, according to this prior art servo valve.
Although the above discussion has been limited to pneumatic servo valves, similar shortcomings are present also in hydraulic servo valves.